Hard X-ray astronomy from balloons

The region of the electromagnetic spectrum between 15 keV and 300 keV is the first high energy photon band available to the astronomer at balloon altitudes. It also represents the point at which a change in experimental technique is required since grazing incidence X-ray mirrors, and, apart from the use of xenon, gas proportional counters become impracticable Thin, actively shielded sodium iodide detectors form the mainstream X-ray detection units. Directionality is achieved by means of honeycomb and modulation collimators. However as suitable position sensitive planes are developed it is possible to anticipate the increasing usage of the coded aperture mask as the key element for fine angular imaging. Balloon-borne hard X-ray telescopes tend to deploy larger sensitive areas than their satellite cousins, and for this reason, with suitably fast timing of the data, may be used to study the classes of objects which exhibit rapid temporal X-ray intensity variations. Spectral studies are also of great astrophysical importance in this range. Apart from the interest in neutron star line emissions, observations in the soft X-ray region have invariably left doubts as to the true nature of the local production mechanisms for specific objects, this is due to the lack of precise definition of the spectral shape of the emissions, particularly at higher photon energies experimental techniques. The usage of balloons and related astrophysical problems are reviewed.